Carbapenemase-producing bacteria isolates are increasingly identified throughout the world (1-8). Their early detection is becoming a major issue in the field of clinical microbiology in order to prevent their spread and preserve the efficacy of carbapenems which are becoming the antibiotics of last resort for treating severe infections. Moreover, carbapenemases are usually associated to many other non-beta-lactam resistant determinants giving rise to multidrug and pandrug resistance. Therefore, due to current population exchange and travel, early recognition of carbapenemase producers is becoming mandatory whatever the antibiotic policy or rate of multidrug-resistant nosocomial infections.
The vast majority of acquired carbapenemases belong to three of the four known classes of beta-lactamases, namely Ambler class A, Ambler class B (metallo-beta-lactamases (MBLs)) and Ambler class D (oxacillinases (OXAs)). These three classes of carbapenemases confer clinical resistance to carbapenems. Consequently, carbapenemase-producing bacteria isolates from these three classes have been involved in nosocomial outbreaks (1,2,7,9,10).
The spread of these three distinct classes of carbapenemases varies significantly worldwide. For example, KPC producers (Ambler class A) are identified mostly in the Americas and Southern Europe, while IMP, VIM, NDM-1 (Ambler class B) are extensively identified worldwide with a main reservoir for NDM-1 in the Indian subcontinent. As for OXA-48-like enzymes (Ambler class D), it was first identified in Klebsiella pneumoniae in Turkey in 2003, where it is highly prevalent (11). However, OXA-48-producing bacteria, in particular Enterobacteriaceae, have also been reported in several other countries such as France, Belgium, United Kingdom, Germany Egypt, India and North Africa suggesting its widespread nature (1,6,12).
As previously mentioned, early recognition of carbapenemase producers is critical to prevent their spread. However, the level of reduced susceptibility to carbapenems may vary significantly among carbapenemase producers making their detection difficult (11,12,13,14,15).
In particular, although OXA-48-like enzymes remain susceptible to extended-spectrum cephalosporins, they confer resistance to penicillins and reduced susceptibility to carbapenems, thereby making the clinical laboratory detection of OXA-48-like producing isolates difficult. In fact, it has been identified in multidrug-resistant isolates, which often accumulate multiple resistance mechanisms, including production of extended-spectrum beta-lactamases (ESBLs). Most carbapenemase producers co-express ESBLs, but several OXA-48-like producing isolates that do not carry ESBL genes may remain susceptible to extended-spectrum cephalosporins (11).
Commercially available media for detecting carbapenemase producers exist. Examples of such media are the carbapenem-containing CHROMagar KPC (CHROMagar company, Paris, France) and the cefpodoxime-containing chromogenic ChromID ESBL medium (bioMerieux, La Balme-les-Grottes, France) which is designed to screen for ESBL producers (16-17). In addition, each medium contains a chromogenic molecule which may contribute to species recognition. The main disadvantage of screening carbapenemase producers using the ChromID ESBL medium is that it cannot detect OXA-48-like producers which are susceptible to cefpodoxime in the absence of co-production of an ESBL (12). In addition, this medium may lack specificity since non-carbapenemase ESBL producers are co-selected on that medium. As for the CHROMagar KPC medium, its main disadvantage is its lack of sensitivity since carbapenemase producers with low level of resistance to carbapenems are not efficiently detected on this medium (12).
Finally, WO 2010/010083 discusses a method for direct detection and differentiation of carbapenem-resistant bacteria in a sample comprising (i) inoculation with said sample of a culture medium comprising at least meropenem and/or ertapenem and at least one chromogenic agent, (ii) incubation of said culture medium under conditions allowing the growth of carbapenem-resistant bacteria, and (iii) detection of colonies formed on said culture medium corresponding to carbapenem-resistant bacteria. The application also discloses a culture medium suitable for use in such a method. However, the culture medium used in this method is neither sensitive enough, nor specific enough to detect carbapenemases producers with reduced susceptibility to carbapenems.